Anywhere except Supre. Supre is advertising language for slut.
My personal favourite is American Apparell, they seriously have the PERFECT tshirts. Really comfy too
The Myer basement is also amazing, cheap prices for brands like Julius and Friends.
If you must, go to a surf store but remember its paying heaps of money so that companys like Roxy can use you as a human billboard...
Cotton On, Sportsgirl, and Dotti are great hope this helps!